Well, first of all I should make the disclaimer that I don't use my Python to syphon the water out. I drain it into a bucket (with a regular aquarium syphon) and just use the Python to fill the tank. I find it easier that way for one person to use it.
But when I have used the syphon on my Python I have noticed it helps to get all the air out of it. I just put the syphon in start filling it, turn up the end so that it empties the air out and then open the syphon up to drain it.
It helps to fill the tank a bit first, (that fills the hose with water and the wider part at the end).
Also it is going to syphon better with the tank above the level of the sink, if it is below it and there is not enough water pressure it may not get enough force to make up for gravity.
Try getting all the air out of the syphon, and if that does not work try a sink that has more pressure or is below the level of the tank.
Good luck